Chinese Backdoor Zegost Delivered Via Hacking Team Exploit
IntroductionZscaler ThreatLabZ has been closely monitoring the usage of Hacking Team's leaked exploits in the wild since July, 2015 and recently uncovered the Emissary Panda APT attack leveraging these exploits. In past two months, we've spotted multiple instances of Zegost Backdoor Trojan installation attempts leveraging Hacking Team's Adobe Flash exploit (CVE-2015-5119) payload. These attacks do not appear to be targeted, but the payload involved in the infection cycle has some resemblance to recent APT payloads from HttpBrowser & the PlugX RAT family.
Attack ChainThe infection cycle starts with a legitimate Chinese real estate and shopping site www[.]kongquechang[.]com, which appears to have been compromised by the attackers and contains an injected script. The injected script will cause a series of redirects leading to Hacking Team's exploit payload as seen in Figure 1. The majority of users were led to the original compromised site following a Baidu search.
|Figure 1: Compromised Chinese real estate & shopping site|
|Figure 3: Hacking Team's Adobe Flash Exploit|
|Figure 4: Embedded shellcode to download & install Zegost|
Zegost Payload IterationsDuring the course of our monitoring we observed the attackers switch the malware payload multiple times.
Payload Type #1 - APT RAT like Zegost Installer
The Zegost payload was being delivered as part of an installer archive, which is similar in structure to the APT RAT PlugX and HttpBrowser as detailed here. The downloaded installer was svhost.exe, which has following file structure:
|Figure 5: APT RAT like Zegost installer archive|
We observed a bug in the persistence module for this payload, which resulted in an incorrect path getting added to the registry entry created by the malware. The result was that upon system reboot the user's machine will no longer be infected with the Backdoor Trojan.
Payload Type #2 - Vanilla Zegost
A modified Zegost payload was being delivered in decrypted form abandoning the installer archive structure. This payload was recently compiled and purports to be a XLLuaRuntime Dynamic Link Library file as seen below:
|Figure 6: Recently compiled Zegost payload|
|Figure 7: Zegost payload file meta data|
Zegost Infection Cycle
- Zegost Trojan drops a copy of itself in the Windows system directory as %SYSTEMDIR%\Autoexec.exe
- It creates the following registry key to ensure persistence upon system reboot:
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\ Nobug = %SYSTEMDIR%\Autoexec.exe
- We also saw an alternate code for achieving persistence by registering a new Windows Service:
ServiceName - DirectX dwx
DisplayName - DirectX Remover auy for Windows(R).
ImagePath - %SYSTEMDIR%\Autoexec.exe
- The Trojan reports the infected system information to a predetermined remote server at the following location:
Domain - www[.]svshot[.]com
Server IP - 123.57.54[.]57
Server Port - 8999
- The Command & Control server domain used in this attack was recently registered and points to a dedicated server hosted on the shady Chinese autonomous system - AS37963 (CNNIC-ALIBABA-CN-NET-AP) as seen below:
|Figure 8: Command & Control server hosted in China|
The same server is also used for the initial Adobe Flash exploitation attempt.
ConclusionHacking Team's exploit payloads remain a popular choice among cyber criminals for weaponizing their payloads. This is the first instance of the Zegost Backdoor Trojan being delivered using Hacking Team's exploit. The Zegost Backdoor payload iterations we observed in this attack chain indicates that the author is testing out new payloads.
Zscaler’s ThreatLabZ has confirmed coverage for these exploits and for the Zegost variants, ensuring protection for organizations using Zscaler’s Internet security platform.
Research by: Deepen Desai, Amandeep Kumar